Coal is considered to be a dirty fuel due in part to its ash and sulfur content. Therefore, considerable effort has been expended to develop clean fuels from coal in the form of gaseous products, liquid products and solid products. The production of relatively clean liquid fuels may be obtained by first producing clean syn gas comprising hydrogen and carbon monoxide thereafter converted by Fischer-Tropsch synthesis to liquid products. On the other hand, the coal may be heated sufficiently to produce naturally occurring oils and the thus obtained oils then treated with hydrogen for desulfurization and quality improvement. However, such a pyrolysis process produces considerable gas and a char product requiring disposal.
Another route for achieving clean fuels from coal involves dissolving coal in a solvent, filtering and catalytically treating the liquid product with hydrogen to remove sulfur and improve the quality of the liquid product and recycling the hydrotreated product as a preferred solvent. There are two primary techniques associated with catalytic hydroliquefaction solvent treating coal known as the Synthoil Process and the H-Coal.TM. Process.
The Synthoil Process slurries crushed and dried coal particles in a portion of its own liquefaction product oil. The slurry is fed to and passed downwardly through a fixed bed of catalytic material in a reactor and contacted therein with turbulently flowing hydrogen. The purpose of this treatment is to liquefy and desulfurize the coal liquid product. A commercially available hydrogenation catalyst comprising cobalt-molybdate on silica activated alumina is a typical catalyst. The product of this operation passes through a high pressure receiver where a gaseous product is separated. The product slurry oil is passed to a low pressure receiver before being contrifuged to remove ash and organic coal residues. Part of the centrifuged oil is recycled for use as slurry oil.
In the H-Coal process, coal is pulverized, dried and slurried with coal derived oil. The slurry thus obtained is mixed with hydrogen, heated and fed to a reaction zone comprising an ebullating bed of hydrogenation catalyst such as a cobalt-molybdenum desulfurizing catalyst. The coal liquid slurry is hydrogenated and converted to liquid and gaseous products. The reaction conditions are normally about 454.degree. C. (850.degree. F.) at 2700 psig. In this prior art known operation, a constant catalyst activity is maintained by adding and withdrawing the catalyst continuously. A slurry of unconverted coal and liquid product is removed from the reactor and recycled to the bottom of the reactor while a slip stream is withdrawn and sent to an atmospheric pressure flash drum. Flashed vapors are passed to an atmospheric distillation tower and the bottoms products are processed in a vacuum tower to obtain vacuum distillate overhead and a vacuum bottoms slurry product. A part of the heavy distillate product obtained from the top of the vacuum unit and the bottom of the atmospheric distillation operation are recycled as liquefaction slurry oil for admixture with dried and pulverized coal particles.
The primary objective of coal liquefaction is to produce a low sulfur and low ash fuel. This is achieved by treating coal at an elevated temperature and hydrogen pressure under conditions promoting hydrogen transfer. This may be carried out either in the presence or in the absence of an added catalyst depending on the type of coal being processed and product desired. It is generally agreed that the production of liquid fuels from coal requires the formation of asphaltenes and hydrogenative conversion of the asphaltenes to an oil product. The asphaltenes which are formed as intermediates in the liquefaction of coal are operationally defined as material soluble in benezene and insoluable in aliphatic hydrocarbons such as pentane and hexane. Thus conversion of coal to asphaltenes takes place at a temperature of about 399.degree. C. (750.degree. F.) or above in the presence of a hydrogen donor material. The conversion of the thus formed asphaltenes to oil product on the other hand takes place at a resonable rate in the presence of molecular hydrogen of high pressure with a hydrogenation catalyst and at relatively high temperature in the range of 371.degree. to 482.degree. C. (700.degree. to 900.degree. F.).
It has been observed by early workers in the field that bituminous coal can be solubilized at elevated temperatures by aromatic hydrocarbons comprising an angular arrangement of rings such as phenanthrene. It is poorly achieved with aromatic hydrocarbons with a linear arrangement of rings such as provided by anthracene. That is, phenanthrene will dissolve up to 95 percent of coal components and anthracene only about 24 percent thereof. It has been further observed that the solvating power of the liquid is improved considerably as its hydrogen content is increased.
There are a large number of potential catalyst types available for use in a coal liquefaction process. Catalysts such as zinc and tin chloride have been shown to be successful, but are expensive, require relatively exotic materials for containment, and cause contamination and disposal problems of materials in the products. Some disposable catalysts resistant to poisoning which do not cause problems with metallurgy or products, appear much too expensive (high metals usage) and do not appear to take full advantage of the metals present. Supported metal oxide catalysts such as is used in the II-Coal system, have been demonstrated to be successful. However, these catalysts are highly susceptible to rapid poisoning by carbon, pore structure blockage, and to slow irreversible poisoning due to contaminant metals and active molybdenum loss. These catalysts are also susceptible to attrition and physical damage within the reaction systems. Diffusional limitations within the pore structure of these catalysts is also a key mechanistic constraint.
Major problems in coal liquefaction by supported catalyst systems are pore blockage by carbon deposition, diffusion and mass transfer limitations of the very large asphaltene/preasphaltene molecules within the relatively small, accessable pores, and metals poisoning (particularly by iron, titanium, and calcium) plus active hydrogenation metals loss.
Pore blockage is the classic initial problem with supported catalysts. Very rapid deposition of metal contaminants and carbon reduce virgin catalyst activity by 80 percent and substantially as soon as the catalyst is subjected to the liquefaction environment. This problem is ameliorated by either increasing the size (and/or number) of the pores, or by reducing particle size of the catalyst to make more surface area accessible near the particle surface. An excellent example of this is shown in FIG. I, where external surface area is plotted versus particle diameter.
Diffusional limitation of large molecules within the catalyst particle is improved by reducing the particle size of the catalyst, increasing the size of the catalyst, increasing the size of the pores, and/or increasing the number of feeder pores. However, for the larger asphaltene and/or preasphaltene molecules, such as shown in FIG. II, it is difficult to conceive of a mechanism by which an active (internal) site could be accessed. Thus, because of the limitations of this setup, it is critical to improved catalytic liquefaction conversion performance as these large molecules are specifically the type of molecules which must be converted.
Metals poisoning and active metals loss can be bypassed to some extent by use of a disposable catalyst system. Metals poisoning of this type has been found to be relatively slow, and thus is not considered a major factor in the liquefaction processed needed to be overcome.